1. Field of the Invention
The present invention relates to a developing apparatus which is used in an electrophotography type or an electrostatic recording type image forming apparatus for developing an electrostatic image on an image bearing body.
2. Related Background Art
Conventionally, developing apparatuses which are provided in image forming apparatuses adopting electrophotography are roughly classified into a developing apparatus which uses a single-component developing method and a developing apparatus which uses two-component developing method.
Most of the developing apparatuses which use the single-component developing method adopt a noncontact type method. A typical developing method includes a jumping developing method using a magnetic toner.
According to the single-component developing method, high quality images can be provided with a simple configuration, but it is difficult to provide color images. According to the single-component developing method using a nonmagnetic toner, color images can be provided, but it is difficult to apply the toner on a surface of a developing sleeve used as a developer carrying body and coating is performed by means of an elastic blade under current circumstances, thereby reducing a security and a durability.
On the other hand, the developing apparatus which adopts the two-component developing method carries a toner to a developing region with a magnetic carrier and develops a latent image by bringing a developer into contact with a photosensitive drum which bears the latent image.
Now, a description will be made of the developing steps of the two-component developing method with reference to FIG. 4.
As shown in FIG. 4, a developing apparatus which adopts the two-component developing method has a developing sleeve 30 which carries a developer, a magnet roller 35 which is magnetic field producing means disposed in the developing sleeve 30 and having a plurality of magnetic poles (pole N1, pole N2, pole N3, pole S1 and pole S2) in a circumferential direction, agitating screws 31 and 32 which comprise an agitating and carrying means, a regulating blade 33 which is disposed as developer regulating means for forming a thin layer of the developer on a surface of the developing sleeve 30, and a developer container 34.
A description will be made here of a developing step of visualizing an electrostatic latent image on a photosensitive drum by a two-component magnetic brush method and a circulating system of the developer.
In such a developing apparatus, the developer which is drawn up by the pole N3 is first formed into a thin layer on the developing sleeve 30 by being regulated its layer thickness with the regulating blade 33 at a process where the developer is carried from the pole S2 to the pole N1 as the developing sleeve 30 rotates.
When the developer formed into the thin layer is carried to the pole S1 which is a main developing pole, an ear is formed on the developing sleeve 30 by a magnetic force.
An electrostatic latent image formed on the photosensitive drum is developed by the developer which is formed into the ear, and then the developer on the developing sleeve 30 is returned into the developer container 34 by a repulsive magnetic field produced by the poles N3 and N2.
As described above, the two-component developing method generally uses magnetic poles which have an identical polarity and are arranged adjacent to one another in a circumferential direction, and once takes a developer after development off a developing sleeve by a repulsive magnetic field produced by these repulsive magnetic poles so as not to allow a hysteresis of an image to remain.
A DC bias voltage and an AC bias voltage are applied to the developing sleeve 30 from a power source (not shown). The two-component developing method generally enhances a developing efficiency and an image quality by applying the AC bias voltage.
As methods to form the latent image on the photosensitive drums, methods are known wherein a photosensitive drum, which is the electrophotographic photosensitive body, is scanned by and exposed to laser beams modulated in correspondence to image signals to be recorded whereby the electrostatic latent image is formed in the form of distributed dots or dots distributed so as to correspond to images. Among these methods, a so-called pulse width modulation (PWM) method which modulates a width of a driving pulse current (duration time) of a laser in correspondence to a shading of an image to be recorded is capable of providing high recording density (that is, high resolution) and a high gradation.
By the way, the developing apparatuses which adopt the two-component developing method have recently been configured so as to be more compact and have longer service lives.
To configure these developing apparatuses compact in regard to a developing process, it is necessary to configure a more compact developer container, a developing sleeve, an agitating and carrying screw, and the like. Furthermore, it is also effective to reduce a space of a developer reservoir in the vicinity of the developer layer thickness regulating portion of the regulating blade 33. For compact configurations of these members and saving of space, it is required to manufacture parts and set various kinds of latitudes with more precision and to make various contrivances.
Furthermore, in order to obtain a longer service life, it is necessary to prevent a toner and a carrier from being deteriorated. For this purpose, the developing apparatus must be configured so as not to compress the developer. The developer container allows the developer to be compressed in the developer layer thickness regulating portion, which is located on the upstream side of the regulating blade 33 in a rotating direction of the developing sleeve in an ordinary configuration, and the developer which is attracted by a developer layer thickness regulating pole is compressed in this region between the developing sleeve and the developer container.
To lessen compression of the developer, it is effective to weaken a force Fr of the developer layer thickness regulating pole which attracts the developer to the developing sleeve (a magnetic attractive force exerted perpendicularly to the developing sleeve). Methods to weaken the force Fr includes methods in which magnetization of the magnetic carrier in a developer is lowered (lowering of magnetization of the carrier weakens a force of rubbing the toner image developed on the photosensitive drum in a developing portion, thereby enhancing an image quality) and methods in which a magnet pattern is configured so that a line of magnetic force emitted from the developer layer thickness regulating pole hardly turns to adjacent magnetic poles and emerges from an outer circumferential surface of a developing sleeve as perpendicularly as possible. As one of the latter methods, there has been proposed a developing method which uses one of repulsive magnetic poles of the developing sleeve as the developer layer thickness regulating pole.
When a repulsive magnetic field is produced by magnetic poles which have an identical polarity and are adjacent in a circumferential direction, a line of magnetic force is emitted from each magnetic pole nearly perpendicularly to the outer circumferential surface of the developing sleeve (in a radial direction of the developing sleeve). In this case, a magnetic flux density changes at a low ratio in a direction perpendicular to the outer circumferential surface of the developing sleeve. As a result, the force which attracts the developer to the surface of the developer sleeve is weakened, thereby lowering a compression degree of the developer. (Such a configuration facilitates drawing up the developer to coat the developing sleeve and is simple when the developer layer thickness regulating pole which is one of the repulsive magnetic poles is disposed lower in a direction of gravity than a stripping pole which is the other magnetic pole for producing the repulsive magnetic field. Furthermore, the configuration may allow a volume of the developer reservoir in the developer layer thickness regulating portion to be decreased and serve for compact configuration of the developing apparatus.) It is needless to say that the service life of the developing apparatus can be prolonged by adopting both methods described above.
However, such a developing apparatus may cause a screw-pitch-like optical density ununiformity to be produced at a rear end of a black solid image when the developing apparatus has the above described configuration, that is, a configuration in which the sleeve has a small diameter and one of the repulsive magnetic poles is used as the developer layer thickness regulating pole. This phenomenon takes place because a mixing ratio between the developer which moves to the developer layer thickness regulating pole after being stripped by the repulsive magnetic field and contains the toner at a lowered concentration (with an image hysteresis) and the developer which is agitated and carried by a screw in the vicinity of the developing sleeve to be supplied to the developer layer thickness regulating pole varies at a screw rotation period in a longitudinal direction of an image region.
The above described phenomenon is likely to take place when the agitating and carrying screw is disposed in the vicinity of the developer layer thickness regulating pole and the developer has a relatively low surface in the vicinity of the developing sleeve. Furthermore, the above described phenomenon is also likely to take place when the magnetic carrier is magnetized at a low degree. This is because the developer is likely to move to the developer layer thickness regulating pole after the developer is stripped by the stripping pole after development, and when the carrier is magnetized at a low degree, the developer is insensitive to the magnetic field and likely to move to the developer layer thickness regulating pole without being stripped by the stripping pole.
As a measure to prevent this phenomenon, it is effective to raise a surface of the developer in the vicinity of the developing sleeve. When the surface is raised, the developer can hardly move to the developer layer thickness regulating pole after being stripped by the repulsive magnetic field after the development due to hindrance by the developer, thereby making it possible to suppress the screw-pitch-like optical density ununiformity.
When, however, an amount of the developer contained in the developer container is simply increased to raise the surface of developer in the vicinity of the developing sleeve, and a continuous durability test of a high image ratio is performed, the result is that the replenished toner is less agitated, an image optical density is increased, and a fog is aggravated because a surface of the developer in the vicinity of a screw which is away from the developing sleeve is also raised, although screw-pitch-like optical density ununiformity was eliminated.
An object of the present invention is to provide a developing apparatus which has high performance to agitate a replenished toner.
Another object of the present invention is to provide a developing apparatus which prevents a fog.
Still another object of the present invention is to provide a developing apparatus configured to allow a carrying screw which receives a developer by the action of a repulsive magnetic field to have a carrying speed lower than a carrying speed of a carrying screw which receives a replenished toner.
A further object of the present invention will be apparent from the following description.